1. Field of the Invention
The invention relates to a method for locating the position of wheels of a vehicle, said wheels being fitted with an electronic module adapted to transmit, to a central unit mounted on the vehicle, signals representative of operating parameters of each wheel.
2. Description of the Related Art
More and more motor vehicles, for safety reasons, have monitoring systems comprising sensors mounted on each of the wheels of the vehicle that are dedicated to the measurement of parameters, such as pressure or temperature of the tires fitted to these wheels, and designed to inform the driver of any abnormal variation of the measured parameter.
These monitoring systems are conventionally equipped with an electronic module, mounted on each of the wheels of the vehicle, incorporating, in addition to the aforementioned sensors, a microprocessor and a radio transmitter, and a central unit for receiving the signals transmitted by the transmitters, comprising a computer incorporating a radio receiver.
One of the problems of such monitoring systems which is necessary to resolve lies in the obligation to associate with each signal received by the receiver of the central unit an item of information relating to the location of the electronic module and hence of the wheel at the origin of this signal, this obligation lasting throughout the service life of the vehicle, that is to say having to be complied with even after changes of wheels or more simply inversions of the position of these wheels.
Currently, a first localization method consists in incorporating an accelerometer in each electronic module and in using a localization technique based on statistical methods consisting in comparing the accelerations of the various wheels in order to obtain an item of information on the respective position of each of said wheels.
This localization method is not however very efficient because it requires notably a considerable running time in order to achieve a discrimination between the various wheels.
A second localization method consists in using at least three low-frequency antennas each positioned close to one of the wheels of the vehicle, and in carrying out a localization procedure consisting in exciting successively each of the antennas by the transmission of a low-frequency magnetic field.
According to this procedure, the electronic module mounted on the wheel situated close to the excited antenna transmits, in response to and in the direction of the central unit, a low-frequency signal comprising a code for identifying said module so that the successive excitation of the various antennas leads to the localization of the electronic modules mounted on the wheels next to these antennas.
The main advantage of such a method lies in the fact that the localization procedure is very rapid and leads to a virtually instantaneous localization after the vehicle has started.
On the other hand, this solution is extremely costly because it requires fitting the vehicle with at least three antennas with all the attendant disadvantages: connecting cables, control amplifiers, etc.
A third localization method consists in determining the position of the wheels based on a comparison of the intensity of the signals received by the central unit from each transmitter. As is notably described in patent EP 0 931 679, this method consists:                in a preliminary phase, in programming the central unit so as to generate, based on the range of amplitude of the signal received from each transmitter, a signature of this signal, then in storing in said central unit each signature and the corresponding position of the wheel,        and, during the use the vehicle, in using a localization procedure consisting in generating, in the central unit, the signatures of the signals received from the transmitters, and in comparing each signature with the stored signatures so as to deduce therefrom the position of the corresponding wheel.        
It turns out, however, that this technique requires the transmission of signals of a relatively long duration, in practice of the order of the time required to achieve one wheel revolution at slow speed, in order to obtain a significant comparison of the signatures.
Accordingly, the localization method cannot be carried out by using, for the purpose of generating the signatures intended to be compared with the stored signatures, the data frames transmitted periodically, when running, via the electronic modules, comprising the code for identifying said electronic modules and the data representative of the measured operating parameters.
The transmission duration of each of these data frames turns out, in effect, to be much too short to allow a significant comparison of the transmitted signal with the stored signatures.
The solution, for the purpose of localization the wheels, therefore consists, according to this method, in using, when the vehicle begins to run, an initial procedure dedicated to this localization, during which the electronic modules transmit not the usual data frames (identification code and measured parameters), but specific long-duration signals consisting, for example, of a carrier.
However, it turns out that, during this initial localization procedure and because of the length of transmission time, frequent collisions occur between the signals originating from the various electronic modules, so that the discrimination of the wheels by the use of this procedure is random or, at the very least, requires considerable running times.